(CNN)The disagreement between some Republicans and Democrats on Russia's intentions in hacking the election rests partially on the lack of agreement between intelligence agencies and the FBI about the conclusiveness of the evidence, officials explained this weekend.

The US intelligence community is increasingly confident that Russian meddling in the American election was intended to steer the election toward Donald Trump, multiple sources have said. That revelation, first reported by CNN a week ago, went beyond the October statement by the 17 intelligence agencies that only said that Russia was seeking to undermine the election, but did not go as far as to say it was to benefit Trump.

The New York Times reported this weekend that part of the reason for the change is that the CIA believes the Russians hacked not only Democratic organizations but Republican groups too, but that they only published documents from Democrats.

Motives unclear

The FBI hasn't concluded that the RNC itself was directly breached, a law enforcement official said Sunday. FBI investigators did find that a breach of a third-party entity that held data belonging to the RNC. But the data appears to have been outdated and of little value to the hackers. The FBI also found that some conservative groups and pundits were hacked. The FBI also hasn't found conclusive evidence to show that it was done to help Trump.

Read More

"At this point, there appears to have been a combination of motivations," one US law enforcement official said. "They wanted to sow discord and undermine our systems. It's clear not even the Russians thought he would win."

Officials familiar with the briefings given to Congress say the CIA assessment wasn't as definitive as has been portrayed in news reports this weekend. The agency developed new information in recent weeks, based on intelligence sources, which prompted a new assessment of the Russian hack. That assessment "leans" toward the view that the Russians were trying to hurt Clinton and help Trump. But the CIA assessment wasn't definitive, the officials said.

Part of the issue is the nature of the CIA and FBI roles in the investigation. The CIA produces raw intelligence, the FBI moves more slowly to reach conclusions based on the intelligence and other investigative work.

Partisan differences

And then there's the partisan views of members of Congress who have been briefed. "Some people in that briefing heard what they wanted to hear. We just gave them the facts and it's up to the policy makers to do what they want with it," a US intelligence official said.

That disagreement was evident when members of Congress were briefed on the latest intelligence findings recently and the FBI officials did not concur, according to the Washington Post.

CNN is told there was at least one high-level briefing by multiple agencies in the past few weeks since the election. In that meeting, CIA informed members of its changed information.

Democrats emerged wanting a more public airing of the information. That led to the letter two weeks ago from Intelligence committee Democrats pressing the President to declassify the information.

Republicans who heard the same information were insistent that it was not so clear-cut.

Some continued to make that argument this weekend.

"The certainty with which it is being portrayed that the intelligence community fingered Russia and revealed multiple attacks -- those are being overblown and put forward with a certainty that doesn't exist," said one Republican congressional aide familiar with discussion among top leadership and committee members. "Absolute work of fiction by whoever is leaking this information to the press."

Incoming Democratic Senate Leader Chuck Schumer said Sunday that lack of agreement among agencies is more reason for further scrutiny.

"The fact the CIA and FBI disagree shows the need for a bipartisan investigation to get to the bottom of this. The investigation should be tough, strong, bipartisan and have access to all materials, classified and not," Schumer said.

Congressional pushback

Back in September, at a briefing of congressional leadership, Republican Senator Mitch McConnell was among those who pushed back at a meeting when presented with intelligence that Russia was trying to steer the elections to Trump, according to a source briefed on the meeting.

Also in September, multiple intelligence agencies briefed House and Senate intelligence committees about information they had gathered showing that Russia was interfering with the elections, according to a congressional source close to the process. The briefers did not explicitly say that Russian hackers were trying to help Trump, but it was a clear from the evidence that they presented that Russia was meddling in the elections -- and Trump was benefiting.

"There was no way that any one could have walked out of there with that the evidence and conclude that the Russian government was not behind this," this source said.

That is different briefing than the one McConnell was at where he reportedly pushed back on the intelligence linking Russia hacks to helping Trump.

But other Republicans, notably some key senators that disagree with Donald Trump's friendlier approach to Russia, are now backing a bipartisan review of the hacking that will coincide with the White House review of election-related hacking going back to 2008.

Senator Lindsey Graham told CNN this week that Republicans need to recognize that this is a bipartisan impact even if it was aimed only the Democratic presidential candidate this time.

"It's pretty clear to me that WikiLeaks was designed to hurt Clinton and it could be us tomorrow, to my Republican friends," said the South Carolina Republican. "What if the Iranians hack into Trump's emails, because they don't like him being tough? As a nation, this is not a partisan issue," Graham said.

House Speaker Paul Ryan has also decried foreign intervention in the election.

"All year, the Intelligence Community, law enforcement, and state and local election officials have been working to ensure that this election was conducted consistent with our long history of free and fair elections. The speaker can not comment on or characterize the content of classified briefings but he rejects any politicization of intelligence matters," said AshLee Strong, Ryan's spokeswoman.